In a liquid crystal display device configured to perform color display, one pixel is typically divided into three sub pixels: a red pixel including a color filter for transmitting red light, a green pixel including a color filter for transmitting green light, and a blue pixel including a color filter for transmitting blue light. The color filters provided in these three sub pixels allow color display; however, about ⅔ of backlight irradiated onto a liquid crystal panel is absorbed by the color filters. Therefore, in such a liquid crystal display device employing a color filter system, a light utilization efficiency may be low. Therefore, field sequential liquid crystal display devices for color display without using a color filter attract attention.
In a typical liquid crystal display device employing the field sequential system, one frame period that is a display period for one screen is divided into three fields. Note that a field may also be referred to as a subframe; however, in a description below, the term “field” will be used in a unified manner. For example, one frame period is divided into a field for displaying a red screen based on a red component of an input image signal (a red field), a field for displaying a green screen based on a green component of an input image signal (a green field), and a field for displaying a blue screen based on a blue component of an input image signal (a blue field). By displaying primary colors one by one as described above, a color image is displayed on a liquid crystal panel. As a color image is displayed in this way, the need of a color filter is eliminated in a field sequential liquid crystal display device. Accordingly, the light utilization efficiency is about three times higher in such a field sequential liquid crystal display device than in a liquid crystal display device employing a color filter system. Thus, the field sequential liquid crystal display device is suitable for achieving high luminance and low power consumption.
Note that, in the present specification, a combination of a data value of a red component, a data value of a green component, and a data value of a blue component is referred to as “an RGB combination”. For example, one RGB combination is “R=128, G=32, B=255”. In this example, the data value of the red component is 128, the data value of the green component is 32, and the data value of the blue component is 255. Typically, the data value is a gradation value.
However, in a liquid crystal display device, an image is displayed by controlling a transmittance of each pixel using voltage (a liquid crystal applied voltage). Relating to this, it takes several milliseconds from a time when writing of data into a pixel (application of voltage) starts to a time when the transmittance is at or higher than a target transmittance in the pixel. Therefore, after a liquid crystal in each field responds to a certain extent, the backlight of each color switches from off state to on state, in a field sequential liquid crystal display device.
Furthermore, in a liquid crystal display device, due to a low response speed of a liquid crystal, in a case of displaying, for example, a video, a sufficient image quality cannot be achieved. Therefore, in the related art, as a solution to the low response speed of a liquid crystal, a driving system called overdrive (over shoot driving) is employed. In the overdrive, in accordance with a combination of a data value of an input image signal of an immediately preceding frame and a data value of an input image signal of a current frame, a drive voltage higher than a gradation voltage determined in advance corresponding to the data value of the input image signal of the current frame, or a drive voltage lower than the gradation voltage determined in advance corresponding to the data value of the input image signal of the current frame, is supplied to the liquid crystal panel. That is, by using the overdrive, a compensation for emphasizing a temporal change (not a spatial change) of the data value is carried out for the input image signal. By employing such overdrive, in an existing liquid crystal display device employing a color filter system, a liquid crystal makes a response such that the transmittance in each frame reaches at or higher than approximately a target value (a target transmittance).
Note that the transmittance of a pixel depends on a liquid crystal state (an orientation state of liquid crystal molecules) in the pixel. In the following description, a gradation value corresponding to the transmittance of a liquid crystal state at each time point is referred to as “a liquid crystal state level”, and the “liquid crystal state level” at a start timing (an end timing for an immediately preceding period) of each period (each field) is referred to as “an arrival gradation value”.
Typically, in a liquid crystal display device that employs the overdrive, the transmittance is assumed to be at or higher than a target value within one frame period. However, depending on a response characteristic of a liquid crystal to be used, the transmittance may not be at or higher than the target value within one frame period. In this case, based on the determination of a drive voltage without considering a change in the transmittance in the immediately preceding frame, a difference between the target value and an actually achieved transmittance may become gradually larger. WO 2003/098588 discloses an invention of a liquid crystal display device capable of providing a solution to this phenomenon. In the liquid crystal display device, arrival gradation data indicating an arrival gradation value (arrival gradation luminance) at the start timing of the current frame is obtained, based on an input image signal of an immediately preceding frame. Then, a drive voltage is determined based on the arrival gradation data and the input image signal of the current frame. By determining the drive voltage in consideration of the arrival gradation value in this manner, an occurrence of an afterimage is prevented and gray levels are correctly displayed, for video images having any type of gradation transition.
Furthermore, JP 2003-502687 T discloses an invention relating to a compensation operation for a color impurity in a color sequential LCD image display device. According to the above-described invention, a signal of each color is compensated based on a signal of a preceding color. For example, in a case where colors are displayed in an order of “blue, green, and red”, a signal of green is compensated based on a signal of blue.